Optical fiber buses offer the potential of extremely high bandwidth capability and electromagnetic interference immunity as compared to bus technology based on electrical transmission. Among the problems in prior art optical fiber bus arrangements, however, is a limitation in the number of locations at which light can be introduced into the bus--referred to herein as "tap sites"--this limitation resulting principally from signal strength loss and reliability issues.
In particular, one prior art approach to the coupling of light into the bus is to sever the bus at each tap site and introduce a discrete coupling device between the severed ends. Disadvantageously, misalignment, insertion loss and other effects at each of the two coupler/bus interfaces created by this "invasive" approach inevitably result in significant dissipation of signal strength.
As a result, the art has actively sought to develop other types of optical bus coupling techniques.
For example, in an arrangement disclosed in U.S. Pat. No. 4,768,854, a severe bend is created at each tap site and light is introduced at the bend through the light fiber buffer and cladding. Anothe approach using bends in the fiber removes the buffer and introduces the light directly into the cladding, as shown in Federal Republic of Germany Patent No. 2,064,503.
Moreover, quite a number of prior art approaches remove both the buffer and cladding at each tap site. For example, U.S. Pat. No. 4,021,097 discloses the coupling of an optical fiber with a slab of light promulgating material. The fiber cladding is removed in the coupling region, and the fiber has a negative curvature which leaves the fiber coupling region under tension. U.S. Pat. No. 4,355,863 discloses the bundling of optical fibers in which a portion of the cladding has been removed. U.S. Pat. No. 4,387,974 discloses an evanescent wave coupler in which two optical fibers which have a portion of the cladding removed are juxtaposed with an inter-leaf film between them. The inter-leaf film secures a constant spatial relationship between the fibers to permit evanescent coupling therebetween. U.S. Pat. No. 4,264,126 discloses an optical fiber coupler in which a pair of optical fibers with their cladding removed are braided in tension and then placed in a coupling solution. U.S. Pat. No. 4,087,156 discloses an optical fiber transmission mixer wherein the cladding material is removed from an intermediate region of a plurality of fibers, and the exposed fiber cores are encapsulated in a matching or slightly higher refractive index material. The encapsulated region is then enclosed in a low index sheath to prevent light from escaping from the mixer.
Disadvantageously, many of the above-mentioned and other non-invasive coupling arrangements known in the art give rise to problems in mechanical reliability. Specifically, bending or other forms of physical stress placed on the fiber may create micro-cracks, ultimately leading to a fracture of the optical fiber. In addition, approaches which remove the cladding expose the tap sites to water and other impurities which can accelerate micro-crack propagation. Moreover, bends at no-longer-needed tap sites cannot typically be fully removed and permanent "microbend losses" may result.